Two categories of autonomous capsules exist. The first category concerns endocardial capsules, which are placed in one of the heart chambers. The second category concerns epicardial capsules which are fixed to the external wall of the myocardium, also known as epicardium.
Endocardial capsules have a cylindrical shape, for example a capsule as shown in FIG. 1 so as to be longitudinally inserted through an in situ implantation accessory, such as a catheter from the venous or arterial system of the patient.
At the end of the capsule a fixing means is present for anchoring the capsule to the desired stimulation site.
An implantable capsule as described in U.S. 2008/0088397 comprises a body housing the main components of the device (electronic circuits, power source, stimulation electrodes, etc.) and a base secured to the body and rigidly supporting means for attachment to the wall, in particular in the endocardial wall.
In the case of cardiac leads, two types of fixation are recognized and traditionally used: the “barbed” fixing is the oldest and is still marginally used, but the leads based on a fixation screw supplanted barbed leads and currently represent majority of the market. They allow a generally robust and effective fixation. The screw has a projecting helical screw which axially extends the body of the capsule and is intended to penetrate into the cardiac tissue by screwing at the implantation site, in the same method as for the conventional screw leads.
However, the fixing of such devices remains a critical issue to the extent that an accidental detachment of the capsule would cause the latter to be released into the heart chamber and then transported by the blood in the venous or arterial system. The complication risk to the patient would be extremely high, and the risk of cardiac system injury that may be generated by the end of the fastening system or any other projecting zones of the implant such as a needle electrode or a projecting ridge.
More than a lead device, an autonomous device meanwhile undergoes stresses and movements generated by the heart wall, as it does not benefit from the axial holding force from the lead body. In addition, it has a certain mass.
To fulfill its permanent anchoring function, the fastening system must also include a function of irreversibility, that is to say, it will only be removed from the heart wall by voluntary intervention of the doctor and according to a predefined procedure, but in no case by repeated movements of the heart.
Additionally, the physician should be able to position the implant and capsule at a location chosen by him, but also to reposition the capsule to another location if the first location does not achieve the expected performance.
Thus, the capsule implant system should be simple and intuitive for the physician, including adoption of implantation procedures close to current practice, which makes use of well known and mastered gestures of the practitioners, including for the implantation of the cardiac leads.
The use of a fixing means comprising an anchoring screw system is in particular described in EP 2818201 (SORIN CRM SAS) and an in situ implantation accessory of such a capsule is disclosed in EP 2818202 (SORIN CRM SAS).
Moreover, these capsules comprise a communication device for communicating with an external device, such as a programmer, by radio frequency or by the human body (HBC) or any other system, and also with one or more other implants, for information transmission and reception.
In order for the capsule, to transmit and receive consistent information, it is necessary that the communication environment is not disturbed, in particular by electric or other fields. In addition to maintaining acceptable lifespan of the capsule after implantation, that is to say about 10 years, the energy cost necessary for communication with an external device should be as small as possible.
Indeed, the energy cost generated by this communication function is significant and must be minimized to ensure maximum longevity of the implant. The link budget between an implant and the peripheral device or devices is paramount to ensure the exchange of data and involves having an electric field as less disturbed as possible and thus link budget with the lowest possible attenuation.
However, it has been observed that the proximity of the metallic fixing means and of the stimulation electrodes causes the formation of an electric field radiation, in particular between first, the attachment means and a first electrode and second, the second electrode.
The electric fields created thereby disrupt communication between the capsule and the external device so as to potentially corrupt the information transmitted between the two devices.
A known solution for reducing the formation of electric fields is to coat the outer surfaces of the fastening means with an insulating coating, for example parylene. However, such a coating with a thickness of about 10 μm does not allow resisting wear due to mechanical movements of the heart. Indeed, during the life of the capsule, namely about 10 years, the capsule will be subjected to approximately 400 million cardiac cycles.
The adhesion of the coating on the fastening means can withstand such mechanical stresses. In addition, cracks in the coating can also be caused by micro-movement of the fixing screw, in particular due to the movement of the heart wall.
Furthermore, it is very difficult to isolate the end of the fastener, this end being pointed in general. Poor insulation of this end results in the generation of leakage current.
Furthermore, the fastening means comprises an anti-unscrewing system having sharp edges, these edges being very difficult to isolate zones.
Finally, a parylene coating has the disadvantage of having a relatively low coefficient of friction which can be detrimental to the attachment system and therefore consequently reduce its effectiveness or result in unscrewing of the capsule.